If you liked the juices in the first place, but got tired of them, just put them away for a while; in a couple of months you might like them all over again.
For juices that taste horrible; there's probably no remedy.
For juices that taste weak, there are several things you can use to try to tweak them up a bit. Menthol (or "Candy Cane" flavoring), vanillin crystals, saline, ethyl maltose, sweeteners, lemon juice, etc. (I only add lemon juice "in the tank" or to very small volumes of e-liquid; it can be great at first but spoils rapidly).
If you have a juice that tastes good, but starts tasting bad partway through a tank, try dripping it instead.
You may need to try different power levels. Some juices don't come out until you vape them at relatively high power, others are much better at lower power levels.
Worst-case, PIF them to someone else. Everybody's tastes differ, and even if you absolutely hate something someone else might really like it. (Trust me on that one....)
I've got a box of bottles that, when I feel bored, I reach in and grab one, to try again. Usually, they still bite, but sometimes I'm surprised, or I discover that I "know" how to fix that one...